Trial 1
Set the transfer case and transmission levers to neutral position and start the engine. Set the engine speed to the speed of the vehicle at which the vibration occurred.
If the vibration persists while the vehicle is stationary, the fastening and condition of the engine mounts should be checked, as they are the cause of vibration.
Trial 2
If no vibration is detected during test 1, place the transfer case levers in neutral, start the engine, engage the transmission in direct gear and set the engine speed to the vehicle speed at which the transfer case vibrates.
If vibration is observed on a stationary car, at such a crankshaft speed, then its cause is a malfunction of the intermediate cardan shaft (imbalance, bending of the fastening bolts or flexible coupling flange, sticking in the constant velocity joint).
Trial 3
If no vibration is found in Tests 1 and 2, proceed to Test 3. Accelerate the vehicle to a speed where vibration occurs and place the transfer case and transmission levers in neutral. If the vibration persists, then it is caused by a malfunction of the front or rear driveshafts (imbalance, jamming of cardan joints) or an imbalance in the center differential.